A conventional armature for an electric motor essentially comprises a core, usually in the form of a lamination stack, a commutator at one end of the core, a spacer at the other end of the core to prevent the windings rubbing against the inside of the motor frame and to fix the length of the motor to give a predetermined end play between supporting bearings, and windings supported by the core and connected by lengths of wire to the commutator. The core, commutator and the spacer are individually mounted as interference fits on the motor shaft commonly with spaces therebetween. Such an armature suffers several drawbacks. Rigidity along the length of the armature is sometimes small, thus allowing the shaft to flex when radial loads are applied to that portion of the shaft outside the motor frame, and also should the armature be out of balance. This flexing reduces the performance of the motor and can lead to complete failure if excessive. Under heavy axial loads movement of, for example, the commutator or spacer along the shaft can cause end play to become excessive. Moreover, torsional strain on the shaft when the armature is used in applications requiring rapid acceleration and deceleration can establish torsional resonances on the armature creating problems of control.
In a conventional armature the forces of acceleration and deceleration when the motor is started or stopped, combined with thermal expansion and contraction of wires due to self-heating, centrifugal forces of rotation, and any external vibrations cause wires lying next to one another to rub together and wear away their insulation layers, thus causing shorting of turns and subsequent loss of performance of the motor. Moreover, the lengths of wire which connect the winding coils to the commutator are suspended in air and having been soldered or welded to the commutator have usually experienced some embrittlement. The forces of acceleration and deceleration, centrifugal force, and external vibrations on these free lengths of wire can cause them to fracture and disconnect the motor.
Moreover, it is not uncommon for localized heating to take place in the windings of conventional armatures causing high temperature rises and failure to occur due to breakdown of insulation.
In a conventional armature, particularly one having an odd number of poles, it is always difficult to ensure an even distribution of the amount of copper in each slot and/or the position of copper in each slot relative to the axis of rotation of the armature. This creates a degree of imbalance resulting in vibration.
It is known to add elements for, inter alia, suppression, which elements are located at or adjacent to the commutator and connected to the commutator or to the wires leading from the winding coils to the commutator. These elements may be fragile and they and the connections to them may suffer serious damage due to the forces of vibration.
Furthermore, in a conventional armature in which joints between the winding wire and the commutator or the aforesaid elements are used some solder flux becomes deposited on surrounding areas. At some time later during operation of the motor this flux may vaporise and recondense on the surface of the commutator or on brush gear causing failure of the motor.
Also, windage losses caused by the drag effect of the uneven slotted surfaces of conventional armature cores and by cavities in the slots themselves can represent a power loss in the motor.
Many of these drawbacks can be removed by encapsulating the windings and the lengths of wire which connect the winding coils to the commutator in a body of plastic. Encapsulation prevents movement between individual turns and wires and thus prevents rubbing or fracture. The body of plastic material also serves to dissipate heat from hot spots in the armature resulting in a more balanced thermal system. It also improves the mechanical balance and rigidity of the armature.
Flush commutators are generally considered to be a bad thing because they can result in increased wear rate of brushes and plastic commonly used to support the commutator segments can be dragged across the commutator segments. It is, therefore, preferred to use a commutator in which slots are provided between the individual commutator segments. However, when encapsulating the windings and lengths of wire which connect the winding coils to the commutator in a plastic body it has been found that plastic material flows along these slots and this is a serious disadvantage.